I would maybe rephrase the multiple choices or the question so as to better encompass the opinions people have on this matter. My opinion, for instance, is disparate from all 3 options you give. Not only that, but you should be more specific: “Does A God exist?” is different from “Does a SPECIFIC God exist?”
1) Well, I certainly do not believe God exists, so this certainly could not be my answer.
2) Well, I do have a belief that there is no God simply for practical reasons. I see no evidence for God and believe that if he were real, there would be evidence. However, when talking about reality, I cannot know for sure that there is no God because you cannot prove a negative (with few exceptions).
3) I very clearly do have an opinion on the matter but it does not fall within your two assigned options.

I am therefore unable to fill out the multiple choice question in an accurate way.

This is a true yes/no question, unlike the above. This is the question I use to separate a theist from an atheist. If the answer is “yes” that is theism. If the answer is anything else (“no”, “maybe”, “I don’t know”) then that is atheism.

I answered no, inferring from the third option which mentions belief that this is a question about belief, and not about knowledge, in which case it becomes analogous to the true yes/no question “Do you believe god exists?”

I don’t believe god exists because I have no evidence that god exists.

The other question you can ask is “do you know god exists?” Questions of knowledge are more difficult. Consider the two questions: “do you believe your father is your father?” and “do you know your father is your father?”

I can easily say “yes” to the first one - I’ve grown up with my father since I was a little boy, etc.

I cannot easily easy “yes” to the second one. That kind of answer would need a DNA test, for example.

(In fact, my own father couldn’t answer “yes” to such a question, when he discovered in his 30s that his father - my grandfather - was not actually his father, but a substitute in order to avoid persecution. My grandfather had to hide his identity during WW2 because, even though he was not Jewish, he was mistaken for being a Jew, and therefore his son, my father, would have been persecuted as well.)